The invention relates to a layer system and a process for producing layer systems for gas sensors in accordance with the generic class of the main claim. Such layer systems are disclosed, for example, in German Patent Specification 28 52 647.
It has been found, however, that a porous covering layer on its own cannot always adequately protect the electrode layer against contaminants from the exhaust gas such as, for example, silicon, phosphorus, zinc, lead and their compounds. EP-A2-0 331 513 therefore proposed to introduce alkaline earth metal oxides onto the surface or into the pores of the covering layer. The gaseous organic or inorganic silicon compounds are thereby trapped and converted into stable compounds which do not impair the operation of the sensor. Although alkaline earth metal oxides are able to trap the contaminant silicon effectively to form refractory reaction products, they are able to trap lead only in the presence of other coreactants such as, for example, silicon and then only to form low-melting reaction products which result in a blocking of the pores in the protective layer and, consequently, in an impairment of the sensor sensitivity.
During the initial motor run after production of a motor vehicle, the contaminant silicon may occur, in particular as a result of escape from sealing parts, whereas, during continuous operation, pollutants such as, for example, lead, phosphorus, zinc, and in exceptional cases also silicon, primarily originating from fuel and oil additives may occur.
EP-A2-0 373 745 furthermore discloses protecting the electrode layer of fuel cells or gas sensors from silicon-containing and/or aluminium-containing compounds in the test gas by depositing one or more oxides, or compounds which form oxides on heating, of the metals from the group comprising Ce, Sm, Mg, Be, Ca, Sr, Ti, Zr, Hf, Y, La, Ce, Pr, Nb, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U.
U.S. Pat. No. 4,272,349 discloses the use of an outer protective layer composed of Al.sub.2 O.sub.3 which acts as a getter for catalyst poisons, in particular for phosphorus.